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Sol–gelsynthesisandcharacterizationof
titaniamonolithwithbimodalporosity
ArticleinJournalofSol-GelScienceandTechnology·May2011
ImpactFactor:1.53·DOI:10.1007/s10971-011-2410-2
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J Sol-Gel Sci Technol (2011) 58:436–441
DOI 10.1007/s10971-011-2410-2
ORIGINAL PAPER
Sol–gel synthesis and characterization of titania monolith
with bimodal porosity
Jing Zhao • Zi-Tao Jiang • Jin Tan
Rong Li
•
Received: 19 September 2010 / Accepted: 22 January 2011 / Published online: 1 February 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Monolithic titania materials with macro-mesoporosity bimodal texture have been prepared through a
template-free sol–gel approach, based on the reaction of
hydrolysis and polycondensation of titanium isopropoxide
promoted by the slow released water from esterification
between acetic acid and methanol under a strong acidic
condition. With the coarsening of the titania oligomers,
phase separation and sol–gel transition processes take place
so as to form a homogeneous gel system that will change
into a monolith after aging, drying and heat treatment. The
synthesized titania monolith possesses a specific surface
area of 77 m2 g-1 (calcined at 350 °C), an anatase with
partly rutile crystallite structure and great mechanical
strength. The synthesis method applied here is simple and
easy to implement as no extra chemical modifier such as
poly(ethylene oxide) (PEO) and formamide is needed to
control the process. The properties of biomodal porous
structure, satisfactory surface area and high mechanical
strength will enable the monolith to be served as a chromatography column to separate phosphorus organocompounds.
Keywords Titania Monolith Sol–gel synthesis Mesoporous Macroporous
J. Zhao Z.-T. Jiang (&) J. Tan R. Li
Tianjin Key Laboratory of Food Biotechnology, College
of Biotechnology and Food Science, Tianjin University
of Commerce, Tianjin 300134, People’s Republic of China
e-mail: [email protected]
123
1 Introduction
As a new material of stationary phase, titania has been
drawing increasing attentions in recent years, owing to its
great mechanical strength, excellent pH stability and
amphoteric ion exchanger characteristic [1, 2]. Most
importantly, the unsaturated titania ions (IV) are strong
Lewis acid sites which have an selective affinity for the
strong electronegative phosphonate group of organocompounds [3] such as phosphoproteins, phosphopeptides,
nucleotides and phospholipids. During the past decade,
considerable efforts have been made to study the enrichment and separation performance of titania for organophosphates [2–7], carboxylates [8] and basic compounds
[9].
The monolithic stationary phase has remarkable superiorities than the particle-packed one, such as low backpressure, high permeability, and high-throughput [2, 10],
which make the synthesis techniques of titania monolith
seriously significant because it will boost enormous
advancement in the field of organophosphates analysis.
When it comes to the fabrication of titania monolith,
template method and sol–gel synthesis as the main routes
have been reported. For instance, urea–formaldehyde (UF)
resin [11] and starch gels [12] were used as the templates to
prepare interconnected titania monoliths. Konishi et al.
[10] applied titanium colloid and poly(ethylene oxide)
(PEO) to synthesize monolithic products with biomodal
framework. Besides, Backlund and coworkers also fabricated TiO2 materials through a sol–gel method [13]. In the
present work, the monolithic titania products with interconnect texture and great mechanical strength were successfully prepared through a simple and reproducible route
based on the sol–gel technique. The heart of the synthesis
was the introduction of esterification reaction between
J Sol-Gel Sci Technol (2011) 58:436–441
437
acetic acid (HAc) and methanol (MeOH) to the system,
which could slowly release water for the hydrolysis and
polycondensation reactions of titanium precursor. Thus the
facile control of the monolith morphology could be
accomplished without adding any polymer or surfactant to
adjust the reaction kinetics.
diffraction (XRD) analysis with Cu Ka radiation (D/max2500, Rigaku Co. Ltd., Japan) was performed to identify
the crystalline phase of the materials calcined at diverse
temperature from 100 to 400 °C and the measurements
were made on the powder specimen prepared by grinding
the monolithic gel.
2 Experimental
3 Results and discussion
2.1 Chemicals
3.1 Gelation factors
Titanium isopropoxide (Ti(OPri)4) as precursor was purchased from Taichang Chemical Co., Ltd. (Tianchang,
China). Analytical grade MeOH was obtained from Kermel
Chemical Reagent Co., Ltd. (Tianjin, China). HAc and
hydrochloric acid (HCl) were both of analytical grade and
from Tianjin No. 1 Chemical Reagent Factory (Tianjin,
China).
In the present study, titania monoliths (shown in Fig. 1)
were obtained from the starting composition containing
Ti(OPri)4, MeOH, HAc and HCl. In order to control the
synthesis reactions to obtain a fine structure material,
considerable efforts have been performed to find out the
relationship between the gel time and the reagent contents.
The gel time was defined as the time elapsed between the
point when all chemicals were added together and the time
when the monolith lost the ability to flow. As displayed in
Fig. 2, the gel time decreased with the increase of HCl/Ti
ratio from 0.3 to 0.6, but increased when the ratio increased
from 0.6 to 0.8 both in the starting composition of Ti/
MeOH/HAc = 1/7/4 and 1/3/4. Besides, the gel time
shortened dramatically when the amount of HAc increased,
but prolonged gradually as the increasing of MeOH
content.
In the synthesis process of titania monolith, Ti(OPri)4 is
firstly hydrolyzed into the one that contains hydroxy groups
on the surface, which will react to the hydroxyl or alkoxy
groups of other titanium molecules, leading to the formation of titania oligomers in the sol. With the reaction
carrying on, the oligomers coarsen gradually and then
cross-link with each other, the process of which is
2.2 Synthesis procedure
In a typical synthesis protocol, 50 mM of HAc was added
to 10 mM Ti(OPri)4 under stirring condition for 30 min to
which a certain amount of HCl was added. After 10 min,
40 mM of MeOH was drop-wisely added to the solution
under vigorous stirring for 30 min, during the process some
water for hydrolysis and polycondensation was slowly
released by the esterification between HAc and MeOH.
Then, the homogeneous solution was poured into a glass
tube that was sealed and allowed to gel at ambient temperature. The resultant gel was aged at the same temperature for 3 days and then dried in ovens at 30, 60 and
100 °C each for 3 days, respectively. What needs to be
taken into consideration was that drying process must be
implemented slowly and at a suitable temperature to make
sure the monolith kept away from crack formation induced
by high capillary forces in small pores inside the monolith.
Finally, the dried gels were heat-treated at 350 °C for 2 h
with a heating ramp of 0.5 °C/min to remove residual
organic compounds. The most noteworthy merit was that
no extra water was needed for the hydrolysis reaction of
titanium precursors, which was slowly released by the
esterification between HAc and MeOH under the strong
acidic condition.
The lm-range morphology of titania monoliths was
observed by scanning electron microscope (SEM; SS-550,
Shimadzu Ltd., Japan). The mesoporous structure was
characterized by nitrogen adsorption–desorption (F-Sorb
3400, APP. Co., Ltd., China) methods. The size distribution of the mesopores was calculated by Barrett-JoynerHalenda (BJH) method and the surface area was obtained
by Brunauer-Emmett-Teller (BET) method. X-ray
Fig. 1 Photograph of titania monolith derived via heat-treatment at
350 °C
123
438
J Sol-Gel Sci Technol (2011) 58:436–441
24
Gel time(h)
20
18
Gel time(h)
25
Table 1 The specific surface area of different samples prepared by
various HCl/Ti ratio in the starting solutions (Ti/MeOH/
HAc = 1:4:5)
20
Mole ratio of HCl/Ti
0.40
0.45
0.50
0.55
0.60
15
Specific surface area (m2 g-1)
58
59
66
66
77
(A)
30
22
16
14
10
12
5
10
0
(B)
0.3
8
0.4
0.5
0.6
0.7
0.8
HCl/Ti
6
4
(D)
(C)
2
2
3
4
5
6
7
8
HAc(MeOH)/Ti
Fig. 2 The relationship between gel time and solvent content. (A) Ti/
MeOH/HAc = 1/7/4, (B) Ti/MeOH/HAc = 1/3/4, (C) Ti/MeOH/
HCl = 1/5/0.5, (D) Ti/HAc/HCl = 1/5/0.5
polycondensation reaction that will reduce the miscibility
between the polar solvent and the polymering titania
oligomers, inducing the initially homogeneous mixture to
separate into solvent-rich and titania-rich phases [10]. HCl
plays an important role in the reactions, it helps to enhance
hydrolysis but retard the polycondensation reaction [2, 14].
When HCl is present, the polymering titania oligomers are
positively charged and simultaneity stabilized by the
electrostatic repulsion in the acidic condition, because the
solution pH is much lower than the isoelectric point of
TiO2 (pH = 5.5–6.0) [14]. Thus, the use of HCl provides
us with an opportunity to control the structural development in the polycondensation stage. Besides, HCl acts as a
catalyst to promote the esterification of HAc and MeOH
(The reaction is apt to take place when the amount of HAc
is superfluous [13].) and the released water is consumed
gradually by the hydrolysis of Ti(OPri)4. As can be seen
from Fig. 2, the gelation time changed when HCl/Ti ratio
increased. The introductions of water, either from HCl
solution or esterification of HAc and MeOH, increases with
the increase of HCl, which promotes the hydrolysis reaction of Ti(OPri)4. At the same time, the number of hydroxyl
groups on TiO2 surface increases so that the polycondensation reaction is accelerated and the gel time decreases in
the range of HCl/Ti ratio from 0.3 to 0.6. Yet, when the
ratio is above 0.6, the process of sol–gel transition is difficult to take place after phase separation and it is prone to
form an inhomogeneity gel system with more TiO2 particles. Moreover, as shown in Table 1, the amount of HCl
also affected the surface area of the monolithic products,
which increased with the increase of HCl amount in the
range of MeOH/Ti from 4 to 5. It was possibly because
large amount of HCl leads to the more complete
123
condensation of Ti–OH on the surface that tends to form a
finer network and hence, resulting in a larger surface area
[15]. The fact should be stated here was that when the HCl
content was small (HCl/Ti \ ca. 0.3), the gels obtained
were straw yellow and translucent, and would change into
pieces during thermal treatment due to the failure in formation of fine skeletons that can resist the stresses.
Titanium alkoxides are more reactive toward water
compared with silicon alkoxides [10], which makes it hard
to control their hydrolysis and polycondensation reactions.
In the previous works of monolithic titania fabrication,
glycerol [1], HAc [13], formamide (FA) [14, 16] and
N-methyl formamide (NFA) [2] were used to control the
reaction process and some improvements have been made.
Backlund et al. [13] have reported that low concentrations
of HAc is a chelating agent to slow down the hydrolysis
and condensation reactions, but above a molar ratio of
HAc/Ti(OPri)4 = 2/1, water is formed in an esterification
reaction between isopropanol and HAc, which in turn
increases the condensation rate of Ti(OPri)4. Therefore, in
the current study, the gel time decreased as the increasing
of HAc/Ti(OPri)4 ratio ranged from 2 to 8.
MeOH is also crucial for the gel formation. Firstly,
MeOH acts as the reactant to provide water for the hydrolysis of Ti(OPri)4 at the presence of HAc and strong acid.
When the amount of MeOH increases, the strengthened
esterification leads to the reinforcement of hydrolysis reaction of titanium alkoxide, so that the number of hydroxyl
groups on the TiO2 oligomers surface increases [14].
Therefore, the compatibility between TiO2 oligomers and
solvent phase increases with the increasing of MeOH. Secondly, MeOH is a good solvent to the TiO2 oligomers, which
will make the system more stable and smaller particles being
obtained as more MeOH cause low depolymerization rate, as
a result the gel time prolongs and the same phenomenon was
also found in the monolithic silica synthesis [17].
3.2 Porous structure
During the process of phase separation accompanied by
sol–gel transition, titania oligomers and polar solvent are
separated to form a homogeneous interconnected gel system, the former becomes the mesoporous skeleton and the
later turns into macropores when the gel is dried at higher
temperature. The SEM image figuring the inner structure of
the monolith is shown in Fig. 3. Although the porous
Cumulative pore volume/cm3g
0.025
3
dV/dD(cm g-1nm-1)
texture was not uniform as the skeleton contained both
particles and bridge-like structure, it was obvious that both
macropores and mesopores were included and interconnect
with each other. Furthermore, this present result was different from the previous study performed by Mir et al. [18],
who synthesized a transparent TiO2 xerogel that was too
weak to resist capillary stresses developed during evaporation and heat treatment and broken into pieces, revealing
the poor inner framework formation. Meanwhile, the
above-mentioned xerogel was characterized with micropores (99% of the pore radius was \1 nm), irregular pore
size distribution and poor mechanical strength, which was
not suitable to be served as a chromatography column.
Nitrogen physisorption measurements have been used to
characterize the texture properties of the monoliths heattreated at 350°C. As shown in Fig. 4II, the pores in the gel
fell into the range of 3–20 nm in diameter as measured by
BJH pore size distribution. The specific surface areas of the
gel obtained by BET method were all ca. 130 m2 g-1 after
100 and 200 °C heat treatment and decreased to ca. 77 and
50 m2 g-1 after 350 and 400 °C treatment, respectively,
ascribing to that stepwise crystallite growth changes the
micropores that yielded high surface area into mesopores
during the process of heat treatment, which was consist
with the result of previous work [2].
From Table 2, the samples prepared by different starting
solutions were found to be equipped with the different
mean pore size, cumulative pore volume and surface area,
that was to say the mesoporous control could be achieved
by regulating the starting composition. Figure 4I and
Table 2 exhibited that the starting solution ratio of Ti/
MeOH/HAc/HCl = 1/3/4/0.6 (Fig. 4IA) decreased in pore
sizes, cumulative pore volumes and BET surface areas
relatived to the one of 1/3/5/0.6 (Fig. 4IB), compared to
which the ratio of 1/4/5/0.6 (Fig. 4IC) exhibited smaller
439
-1
J Sol-Gel Sci Technol (2011) 58:436–441
0.020
0.015
0.010
0.005
Ι
0.8
(A)Ti:MeOH:HAc:HCl=1:3:4:0.6
(B)Ti:MeOH:HAc:HCl=1:3:5:0.6
(C)Ti:MeOH:HAc:HCl=1:4:5:0.6
(D)Ti:MeOH:HAc:HCl=1:4:5:0.5
0.7
(B)
0.6
(D)
(C)
(A)
0.5
0.4
0.3
0.2
0.1
1
10
100
Pore dianeter/nm
0.000
0
20
40
60
80
100
Pore dianeter/nm
Fig. 4 Mesoscale characterizations of titania monoliths calcined at
350 °C. BJH pore size distribution for the Ti/MeOH/HAc/HCl ratio
of I (A) 1/3/4/0.6, (B) 1/3/5/0.6, (C) 1/4/5/0.6, (D) 1/4/5/0.5 and II 1/4/
5/0.6
mean pore size and cumulative pore volumes but larger
BET surface areas. Furthermore, the mesoporous properties
were different between the HCl/Ti ratio of 0.6 and 0.5, the
former possessed a small mean pore size, a larger surface
area and a sharper pore size distribution than the later.
All the above observations indicated that the starting
composition was important for the mesoporous character.
HAc prefers to enlarge the pore size and cumulative pore
volume probably because the more HAc amount the more
large particles are formed. MeOH is likely to decrease the
pore size but increase the surface area due to its property of
good solvent for the TiO2 oligomers that will be polymerized with each other in a relative small particle size.
The complex effect of HCl on the porous properties has not
been definitely demonstrated and we inferred that much
HCl amount contributes to arrange the pores and make
them fall into a sharp distribution. In addition, the large
shrinkage (ca. 50%) was found to happen during the gels’
drying process, which was caused not only by the innersolvent evaporation but also by the condensation of titanium species with unreacted alkoxy or hydroxy groups
[14]. However, only a slight increase in the shrinkage was
observed when the dried gel was calcined at 350 °C. It may
be attributed to that the fully improved crystallinity of the
gel has been obtained during the aging treatment, which
grows minimal when it is heat-treated. Furthermore, the
large shrinkage ranged from 51 to 57% results in the
increase of concentration of TiO2 and hence the enhancement of the mechanical strength of the monoliths.
3.3 Crystal structure
Fig. 3 SEM image of titania monolith prepared with mole ratio of
Ti/MeOH/HAc/HCl = 1/4/5/0.5 after calcination at 350 °C. The
scale bar corresponds to 2 lm
The X-ray diffraction (XRD) measurements were performed for the diverse heat-treated (100–400 °C) gels
123
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J Sol-Gel Sci Technol (2011) 58:436–441
Table 2 Texture properties of TiO2 gels derived via heat treatment at 350°C
Mole ratio of Ti/MeOH/
HAc/HCl
Mean pore size
(nm)
Cumulative pore volume
(ml g-1)
Specific surface area
(m2 g-1)
Linear shrinkage (%)
Dried
(100 °C)
Heat-treated
(350 °C)
1/3/4/0.6
17.16
0.20
53
55
57
1/3/5/0.6
19.24
0.31
65
53
55
1/4/5/0.6
1/4/5/0.5
13.32
15.14
0.24
0.25
77
66
53
48
56
51
The linear shrinkages were estimated by the length of TiO2 rod from the initial gel to the dried and heat-treated ones
Intensity / cps
prepared with the molar ratio of starting composition being
Ti/MeOH/HAc/HCl = 1/4/5/0.6 to evaluate the crystallinity of the gel skeleton. According to the XRD spectra in
Fig. 5, anatase-type TiO2 was found to be the main crystallinity as the most intense diffraction peak of anatase was
can be seen at the angle 2(&25°. The growth of crystalline
anatase TiO2 phase became increasingly prominent as the
heat treatment temperature increase due to the annealing
temperature can improve the crystallinity [18]. The present
samples were much well crystallized because the anatase
phase was prominent even at 100 °C calcining temperature,
which was different from the result that TiO2 was amorphous when it was heat-treated at low temperature [13, 18].
Thus, we presumed that the structural rearrangement from
amorphous into a more stable anatase crystalline phase was
accomplished at lower temperature when the drying process
was performed, which was probably because the acidic
condition that made the titanium oxo or hydroxo bridges
broken to enhance the dissolution and reprecipitation [14].
Meanwhile, a relative small peak at the angle 2(&27°
was also can be found in Fig. 5, showing that partly rutile
(D)
(C)
(B)
(A)
10
20
30
40
50
60
70
80
90
100
2θ / degree
Fig. 5 XRD pattern of TiO2 derived via the molar ratio of Ti/MeOH/
HAc/HCl = 1/4/5/0.6 after diverse heat treatment. (A) 100 °C,
(B) 200 °C, (C) 350 °C, (D) 400 °C
123
phase accompanying the anatase was present in the alkoxy-derived TiO2. However, the phase transition from
anatase into rutile was reported to mainly occur at high
heating temperature above 600–700 °C [2, 13, 14, 19].
The fact that rutile phase was present in the gel after
heat-treatment (100–400 °C) may be contributed to either
the inhomogeneous particle growth [15] accompanied by
partly bridge-like structure or the low pH (ca. 0.15)
starting composition that made titania be apt to form a
anatase–rutile mixed phase [20]. We concluded that
the gels’ crystallinity condition was co-determined by the
heat treatment temperature and the synthesis method. The
esterification reaction between HAc and MeOH to provide water for the hydrolysis and polycondensation
reactions of titanium precursor under a strong acidic
condition make the prepared gels prone to form a more
prominent anatase crystallinity accompanied by small
rutile phase.
4 Conclusion
Titania monolith with meso-macropores network has been
successfully prepared by hydrolysis and condensation of
Ti(OPri)4 which was achieved by the slow released water
from the esterification between HAc and MeOH under a
strong acidic condition. The interconnected gel skeleton
consisted of anatase-type TiO2 nanocrystals companied by
partly rutile phase under the diverse heat treatment from
100–400 °C. The typical as prepared sample was equipped
with biocontinuous porous texture, relative narrow pore
size distribution centered at 13 nm and surface area of
77 m2 g-1. The monoliths were found to have great
mechanical strength ascribed to that the TiO2 concentration
was enlarged as the linear shrinkage was ca. 55%. The
mesopore characters could be controlled by adjusting the
starting compositions. The properties of high mechanical
strength, bimodal porous structure and relative high surface
area will make the monoliths capable to be served as
chromatography columns to separate chemical and biological materials.
J Sol-Gel Sci Technol (2011) 58:436–441
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 20875069) and the
Science Foundation for Young Teachers of Tianjin University of
Commerce (Grant No. 090107).
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